Water and a giant hunk of bedrock 200 feet beneath Columbus has bogged down the city’s giant boring machine, leading to a two-year delay and a $29.5 million cost overrun.Wow, 200 test holes is a bunch. I bet some engineer is feeling pretty bad about this project.
The machine, nicknamed “Marsha,” is boring a 4.5-mile, 23-foot-wide tunnel that was originally to be completed this month. The tunnel is supposed to catch sewer overflows that would otherwise spill into the Scioto River during storms. The project was originally estimated to cost $342 million....
The $26 million custom machine, like something out of science fiction, was built to drill through mostly dry bedrock. But city engineers’ estimate of water levels didn’t account for millions of gallons of water that travel underground, below the Scioto River.
Instead of grinding through dry rock, Marsha was choking on a water/rock/mud mixture called slurry, which is kind of like wet concrete.
The 500-foot-long drilling machine is a cross between a submarine and a huge mechanical worm with a 95-ton cutting wheel on the front. The machine was built in Germany to digest dry rock by sending it back through a crushing screw shaft in its belly, depositing the crushed rock on conveyers that take it to the surface.
The Kenny Construction Co., based in Illinois, and the Japanese-based Obayashi group designed Marsha based on specifications provided by the city....
Marsha was to be able to drill through and remove about 68 feet of dry rock a day, or about 40 feet a day if there was water. The steel cutters on the machine have to be replaced about every 500 feet.
But the unexpected amount of water in the ground forced it to remain closed. The pipes weren’t big enough to handle the slurry because large chunks of rock would make it through the grinder.
For much of the past two years, Marsha has drilled about 25 feet per day.
At one point when the cutters needed to be replaced, the city had to hire expert divers for $1 million. The divers could work only for minutes at a time in the high-pressure underground water to replace the cutting wheels.
Davies said engineers then spent nearly a year reconfiguring Marsha with new, larger pipes and ripping out the conveyer system to add another large screw that would break down the rock even more.
Dax Blake, an engineer and the city’s sewers and drains administrator, said the city had drilled about 200 holes to test the ground before the project began.
“You never really know what’s that far underground, so all we could do was take those samples and calculate out what was down there,” he said.